The invention is directed to cartridges and corresponding chambers for use with firearms of various sizes, and preferably with rifles and long guns having a barrel length greater than about 18 inches.
Firearm technology has advanced from the early muzzleloader wherein black powder and projectiles where separately loaded into the muzzle of a firearm barrel. Modern firearms use a cartridge which includes a case, housing a propellant, a primer, and a projectile. Cartridges have greatly reduced the frequency of misfires that were commonly experienced with case-less ammunition. For rifle and handgun ammunition the case is typically but not necessarily metallic, such as brass, aluminum or steel. A case may or may not utilize a shoulder disposed below a case neck. The case neck retains a projectile. Configured with a shoulder, the case body may have a larger interior diameter than the projectile. For shotgun ammunition, the case is typically paper or plastic with a metal head and is called a shell. The primer is the ignition component which is affixed to the case in a manner to be in communication with the propellant through a flash hole. The primer includes pyrotechnic material such as metallic fulminate or lead styphnate and may be located within the center base of the case or on a rim. Larger cartridges may utilize a “spit tube” extending along the centerline of the case as an ignition aid.
The rear portion of a firearm barrel includes a chamber which is designed to receive the cartridge. The firearm includes a firing mechanism that drives a firing pin or an electrical charge to ignite the pyrotechnic material in the primer. A combustion process is initiated within the cartridge when the primer ignites. Hot high-pressure gases and particulates are produced by ignition of the primer pyrotechnic. The gases exit through a flash hole or holes into the case, which contains the propellant and trapped air. The propellant is typically a combustible powder having various configurations of granules or grains. The propellant and entrained air not ignited by the primer-blast is compressed into a solid mass having the characteristics of a very viscous fluid having excellent compressive strength but little shear strength.
Firearm cartridges are divided into two basic types, straight-walled and bottlenecked, which are distinct in shape and function. Straight-walled cases are so named because they have a cylindrical or slightly tapered shape with an inside diameter equal to or slightly greater than the projectile diameter. Bottlenecked or shouldered cases are so named because they taper from a base to a frusto-conical shoulder and neck which holds the projectile.
The straight-walled and bottlenecked cartridge shapes have distinctly different combustion characteristics and efficiencies. In the straight-walled case, propellant that was not initially ignited by the primer, burns from the aft, or flash hole, end forward with most of the propellant following the projectile into the barrel bore. The propellant along the case wall, although sheared away from the case wall by projectile movement, may not ignite because the case wall has up to 400 times the thermal conductivity of the propellant and significantly greater specific heat. This has the effect of cooling and quenching ignition at the case wall in addition to causing significant heat loss to the cartridge case and gun chamber.
Acceleration losses are high and powder burn rates must be very fast to minimize such losses. Any propellant not consumed before the projectile leaves the muzzle will be expelled and cannot contribute to projectile acceleration. Heat losses caused by burning propellant in the barrel are very high.
The bottlenecked or shouldered case is somewhat more efficient. As propellant is ignited at the primer flash hole or holes, a shock wave moves through the propellant that compresses and heats the propellant. The shock wave is partially reflected off the case shoulder toward a central interior portion of the case. As pressure behind the shock wave begins to move the projectile, the propellant plug approximately the diameter of the projectile is sheared away from the body of the charge. Ignition along the resulting shear surface is rapid because only an infinitesimal gas path out of the shear layer exists causing a rapid pressure and temperature buildup. The portion of the propellant plug which is exposed to the case neck can only burn from the aft end forward due to the quenching effect of the case neck and later the barrel bore.
Burning rates for propellants used in the bottleneck case must be slower because of the additional burning surface of the propellant plug and exposed propellant shear surface. In the region where unignited powder exists, exposure of the case wall to combustion gas occurs when the propellant is consumed. As this material burns forward from the base and through from the interior surface, more of the case is exposed to direct heating, therefore, heat loss increases. Thus, heat and acceleration losses are lower with the bottleneck case but are still excessive. Ballistic calculations utilize empirically derived coefficients drawn from the vivacity curve, such as progressivity, regressivity, and progressivity-regressivity rollover coefficients to define the pressure in a cartridge as a function of time or bullet movement. However, the burning surfaces of the propellant are not quantitatively defined.
In firearm manufacturing, it is desirable to increase the propulsion of the projectile for improved velocity range and accuracy. Projectile velocity and propulsive efficiency have been increased through the use of high energy smokeless powders. Other improvements have resulted from increased case capacity, improved primer design, and better metallurgy for cases and firearms with higher operating pressures. The shape of the case has also been altered, as discussed above, to create the bottlenecked case that increases case capacity to reduce heat and acceleration losses. Improvements thus far have relied upon empirically derived coefficients that do not accurately model pressure over time. Thus, such improvements fail to provide an optimal configuration.
In improving a cartridge several design parameters must be considered within the framework of the combustion process described above. One parameter is to minimize heat losses to the cartridge case, projectile base, and gun barrel. This may be done by protecting cartridge surfaces from combustion heat where possible. Heat losses may also be minimized by reducing the interior surface area of the case as much as possible for the required propellant volume. Another parameter is to maximize the pressure-time integral of propellant combustion within pressure limitations of the firearm design. A further parameter is to complete as much combustion as possible within the cartridge case to minimize heat loss and damage to the firearm barrel. Yet another parameter is to minimize mass and acceleration of uncombusted propellant to conserve combustion energy.
Thus, it would be an advancement in the art to improve the propulsive efficiency of a cartridge. It would be an advancement in the art to increase bullet velocity for a given amount of propulsive medium, such as gun powder. It would also be an advancement in the art to be able to calculate pressure as a function of time directly from propellant burn rates and surface areas without resorting to empirically derived coefficients. Such a cartridge and case-less gun chamber design is disclosed herein.